Heat film-air mass-flow meters are used in motor vehicles, for example, in intake or charger apparatuses of the internal combustion engine. With the chemical process of combustion, the amount of air depending on the mass ratio of fuel and required for combustion is high. Therefore, in the intake apparatus/charging apparatus of the internal combustion engine, the mass flow rate of the intake air/charging air is determined. In addition to volume or dynamic pressure methods, methods that measure mass flow of the air current are also used.
In the intake apparatus of internal combustion engines, today micro-mechanical heat film-air mass-flow meters, which work according to the thermal principle, are also used in part. These are very small and require little space. Heat and sensor resistors are mounted with these very small flow-meters as thin Pt-layers on a chip, which can be made from silicon and serve as a support. On the chip, a mechanically thinned region in the form of a support membrane is applied for thermal decoupling with this type of flow meter. The chip is manufactured by sawing a silicon wafer. The chip encompasses a silicon frame, if necessary, with supports and a sensor membrane with sensor resistors and heat resistors. The center surface region of the membrane represents the sensor element.
A heat resistor of the flow meter is regulated by one or more closely adjacently mounted heat temperature sensors, as well as an air temperature sensor, which is located in an edge region of an SI chip with a large material thickness. With these heat film-air mass-flow meters, the heat flow is not used as the output signal, rather the temperature difference of the gas-type medium to be measured, determined by two temperature sensors, such as, for example, the combustion air, is used. One of the temperature sensors lies in front, that is, upstream, and one of the temperature sensors lies behind the heat resistor in the flow direction of the gas flow to be measured. In contrast to the heat flow, this output quantity again provides the flow with the correct sign, also when it is not in a linear manner.
With applications of a micro-mechanical heat film-air mass-flow meter, under predetermined conditions, contamination of the sensor chip with water, dust particles, or, in the case of use with an internal combustion engine, with motor oil can take place. These types of contamination can affect the sensor signal of the sensor element so markedly, that it no longer lies in the region of the specified tolerances and a premature replacement of the sensor element or the entire apparatus can be required. With cramped proportions in the internal combustion engine, which leads to a minimal distance between the mouth position of a crank housing ventilator in the intake apparatus and the position of the heat film-air mass-flow meter, contamination of the sensor element of the heat film-air mass-flow meter is caused by a deflector grid on the leading edge and an oil condensation grid on the trailing edge. With these features, an increase in robustness of the sensor element of the heat film-air mass-flow meter can be achieved, which however, might not completely exclude a characteristic line drift on the sensor element of the heat film-air mass-flow meter, based on the contamination by oil impingement.